Published Patent App. US 2013/0296398 reports that the combination of phytocannabinoid cannabidiol (CBD) with an anti-epileptic barbiturate drug, which solely enhances GABAergic inhibition, such as phenobarbital, appears not to provide any benefits in treating epilepsy when tested in a pilocarpine model.
Charalambous et al in BMC Veterinary Research 2014, 10:257 report on studies done to treat canine epilepsy using phenobarbital and other drugs, but baseline variations, study designs, and sources of bias preclude definitive recommendations.